None.
This invention relates to emitter follower circuits. In particular, the present invention relates to stabilization of an emitter follower input stage of an integrated circuit differential amplifier to provide stabilized high band width performance.
An emitter follower circuit is well known to be potentially unstable when its output is capacitively loaded. This potential instability is the result of the input impedance of the emitter follower having a real part which becomes negative as a function of frequency. This can cause oscillation or instability when certain forms of source impedance are connected to the input terminal of the emitter follower.
An example of a circuit in which performance can be limited by potential instability of an emitter follower is a preamplifier connected to a magneto-resistive (MR) sensor read head of a disc drive. Source inductance, such as the inductance associated with the leads between the MR sensor and the preamplifier, can result in oscillation or instability of the emitter follower input stage of the preamplifier.
One approach for stabilization of an emitter follower is described in the Peterson U.S. Pat. No. 3,860,836. The load impedance of the emitter follower includes inherent load capacitance which provides a negative real term at some frequencies in the input impedance viewed from the base of the emitter follower transistor. An RC network is connected to the base of the emitter follower so that the network is effectively in parallel across the base and emitter. The disadvantages of this stabilization technique described in the Peterson patent include higher noise and a limited frequency range over which the stabilization is effective.
There is a need for improved stabilization of emitter follower stages of differential amplifiers used in high band width applications. An example is a preamplifier for an MR sensor in a disc drive, where stabilization is required at frequencies ranging from about 500 megahertz to about 10 gigahertz.
The present invention is a differential amplifier having a pair of emitter follower input transistors forming an emitter follower input stage. A stabilization network is connected to the collectors of the emitter follower input transistors to provide negative common mode feedback at frequencies where the real part of the input impedance of the emitter followers becomes negative.